The Triton has been wriggling grotesquely in our grasp while we have made him our text, and, now he is restored to his vase, plunges to the bottom with great satisfaction at his escape. This water-snail, crawling slowly up the side of the vase, and cleaning it of the green growth of microscopic plants, which he devours, shall be our representative of the second great division—the Mollusca. I cannot suggest any obvious character so distinctive as a backbone, by which the word Mollusc may at once call up an idea of the type which prevails in the group. It won't do to say "shell-fish," because many molluscs have no shells, and many animals which have shells are not molluscs. The name was originally bestowed on account of the softness of the animals. But they are not softer than worms, and much less so than jelly-fish. You may know that snails and slugs, oysters and cuttlefish, are molluscs; but if you want some one character by which the type may be remembered, you must fix on the imperfect symmetry of the mollusc's organs. I say imperfect symmetry, because it is an error, though a common one, to speak of the mollusc's body not being bilateral—that is to say, of its not being composed of two symmetrical halves. A vertebrate animal may be divided lengthwise, and each half will closely resemble the other; the backbone forms, as it were, an axis, on either side of which the organs are disposed; but the mollusc is said to have no such axis, no such symmetry. I admit the absence of an axis, but I deny the total absence of symmetry. Many of its organs are as symmetrical as those of a vertebrate animal—i.e. the eyes, the feelers, the jaws—and the gills in Cuttlefish, Eolids, and Pteropods; while, on the other hand, several organs in the vertebrate animal are as unsymmetrical as any of those in the mollusc, i.e. the liver, spleen, pancreas, stomach, and intestines.[1] As regards bilateral structure, therefore, it is only a question of degree. The vertebrate animal is not entirely symmetrical, nor is the mollusc entirely unsymmetrical. But there is a characteristic disposition of the nervous system peculiar to molluscs: it neither forms an axis for the body—as it does in the Vertebrata and Articulata—nor a centre—as it does in the Radiata—but is altogether irregular and unsymmetrical. This will be intelligible from the following diagram of the nervous systems of a Mollusc and an insect, with which that of a Star-fish may be compared (Fig. 18). Here you perceive how the nervous centres, and the nerves which issue from them, are irregularly disposed in the molluscs, and symmetrically in the insect.
But the recognition of a mollusc will be easier when you have learned to distinguish it from one of the Articulata, forming the third great division,—the third animal Type. Of these, our vases present numerous representatives: prawns, beetles, water-spiders, insect-larvæ, entomostraca,
- ↑ In some cases of monstrosity, these organs are transposed, the liver being on the left, and the pancreas on the right side. It was in allusion to a case of this kind, then occupying the attention of Paris, that Molière made his Medecin malgré Lui describe the heart as on the right side, the liver on the left; on the mistake being noticed, he replies: "Oui, autrefois; mais nous avons changé tout cela."
